RADIO EVIDENCE FOR ELECTRON ACCELERATION IN A CUSP-LIKE STRUCTURE

In the spectrum of the type IV/II radio burst of 1994 July 18 we found a rare superposition of fast and slowly drifting features in the freq uency range 100-300 MHz about 1 min before the onset of the shock indu ced meter wave type II burst. We take this as signatures of the passag e of an MHD-like disturbance and the simultaneous injection of two con tradirected electron beams. These beams are manifested as an ordinary type Ill burst and a reverse drift burst starting at the same frequenc y. Remarkably, the reverse drift burst is visible only due to its abso rbtion trace in an underlying continuum patch. We argue that the super imposed burst features are emitted from a common source which covers t he top of a closed magnetic field structure and the immediately superp osed part (cusp) of the helmet streamer configuration. The radio sourc e is situated immediately at the electron acceleration site.